1. Technical Field
The present invention relates generally to the fabrication of semiconductor integrated circuits, and more specifically to ball grid array packages and chip scale packages.
2. Background Art
In the electronics industry, the continuing goal has been to reduce the size of electronic devices such as camcorders and portable telephones while increasing performance and speed. In the past, integrated circuits were packaged in lead-frame packages, but the packaging technology has been moving towards ball grid array (BGA) packages and chip scale (CSP) packages as higher performance packages are required.
Lead-frame packages generally have small metal strips or leads, which extend from the undersides of the packages and which are soldered to the printed circuit boards used in the various products. They have been used for a long period of time in integrated circuit packaging history mainly because of their low manufacturing cost and high reliability. However, as integrated circuits products move toward being both faster and smaller in size, the traditional lead frame packages have become gradually obsolete for many small, high performance-required packages.
BGA packages are widely used for integrated circuit chips that have higher numbers of input and output connections, and which need better electrical and thermal performance than lead-frame packages. Balls of solder are formed on the undersides of the packages and are melted to connect the packages to the printed circuit boards. These packages are generally used in high performance central processing unit (CPU) and video-graphic chips.
The CSP packages are generally used for integrated circuits having 100 or more input/output pins and a large integrated circuit size. Generally, packages that are smaller than 120% of the size of the integrated circuit are typically referred to as CSP packages. The CSP packages have been widely used in mobile products where the footprint (the size of the package on a substrate), package profile, and package weight are of major concern. CSP packages can also be BGA packages as well as other small packages such as land grid array (LGA) packages with a single grid array on the bottoms, and small outline non-leaded (SON) packages with multiple grid arrays on the bottom.
A BGA or CSP package generally consists of a substrate having openings for the ball grid array or connections. The substrate is generally of a polyimide. On the substrate are a plurality of electrical traces including ground traces and power traces. On the substrate over the conductive traces, which are generally of copper, there is an epoxy, which bonds an integrated circuit die to the substrate.
A single metal layer BGA or a CSP package does not have a separate ground plane or closed loop ground. Where a netlist (the list of the required connections) requires a connection between opposite sides of an integrated circuit to power, the power connection is made directly across the substrate using the single metal layer by patterning and etching the metal to the desired cross-connect configuration. Since the ground must be insulated from the power, geometric constraints prevent the use of the single metal layer for a ground cross-connect between opposites sides of an integrated circuit; e.g., the cross-connects cannot intersect.
A second metal layer entails additional cost and complexity because of the additional processing and layers required of insulation, metal, and epoxy. In addition, the second metal layer would result in a substantially increased height for the package.
Solutions to these problems have been long sought, but have long eluded those skilled in the art.